The invention disclosed herein relates generally to inductive components, and, more particularly, relates to inductive components having inductances responsive to magnitudes of DC bias currents.
FIGS. 1-7 illustrate an example of a conventional inductor 100. The inductor has a first E-core 110 and a second E-core 112, which are inserted into a passageway 122 of a bobbin 120. Each E-core comprises a ferrite material or other suitable material. The bobbin has a first outer flange 124 and a second outer flange 126. In the illustrated example, the bobbin further includes a middle partition 130. A first coil 132 is wound around the bobbin between the first outer flange and the middle partition. A second coil 134 is wound around the bobbin between the second outer flange and the middle partition. Other embodiments may include additional partitions and additional coils. Other embodiments may also omit the middle partition and have only a single coil wound between the two outer flanges. The first and second coils are electrically connected to a plurality of pins 140 that extend from a first pin rail 142 and a second pin rail 144. The first pin rail is proximate to the first outer flange; and the second pin rail is proximate to the second outer flange.
The first E-core 110 has a middle leg 150, a first outer leg 152 and a second outer leg 154. The three legs extend perpendicularly from an inner surface 158 of a main body 156 of the first E-core. The second E-core 112 has a middle leg 160, a first outer leg 162 and a second outer leg 164. The three legs extend perpendicularly from an inner surface 168 of a main body 166 of the second E-core.
The middle leg 150 of the first E-core 110 is inserted into the passageway 122 of the bobbin 120 such that the inner surface 158 of the main body 156 of the first E-core is proximate to an outer surface 170 of the first outer flange 124. The middle leg 160 of the second E-core 112 is inserted into the passageway of the bobbin such that the inner surface 168 of the main body 166 of the second E-core is proximate to an outer surface 172 of the second outer flange 126. The inner surfaces of the main bodies of the E-cores may abut the outer surfaces of the outer flanges as shown; or the inner surfaces of the main bodies of the E-cores may be spaced apart from the outer surfaces of the flanges by a small distance. The outer legs 152, 154; 162, 164 of the two E-cores are positioned along the outer boundaries of the bobbin. When abutted as shown in FIG. 3, the two E-cores have an overall length L1 from an outer surface 174 of the main body of the first E-core to an outer surface 176 of the main body of the second E-core.
The middle legs 150, 160 of the two E-cores 110, 112 have a common width W1 between a respective first side surface 180 and a respective second side surface 182. The middle legs have a common height H1 between a respective lower surface 184 and a respective upper surface 186. The passageway 122 has a width W2 between a first inner side wall 200 and a second inner side wall 202. The passageway has a height H2 between an inner lower wall 204 and an inner upper wall 206. The width W2 of the passageway between the first and second inner side walls may be approximately the same as or slightly greater than the width W1 of the middle legs. Similarly, a height H2 of the passageway between the inner lower wall and the inner upper wall may be the same as or slightly greater than the height H1 of the middle legs. As shown in the cross-sectional views of FIGS. 3 and 7, the middle legs 150, 160 of the two E-cores 110, 112 may fit snugly within the passageway 122 with little or no lateral movement or vertical movement. In other embodiments, the widths and the heights of the middle legs may be selected such that the middle legs fit loosely within the passageway. In other embodiments, the middle legs may be constrained by crushable ribs (not shown) extending from the walls of the passageway.
In the illustrated embodiment of FIGS. 1-7, the middle leg 150 of the first E-core 110 is shorter than the first outer leg 152 and the second outer leg 154 by a first length difference LD1 (FIG. 6) such that an end surface 210 of the middle leg is closer to the inner surface 158 of the main body 156 of the first E-core than a respective end surface 212 of the first outer leg and a respective end surface 214 of the second outer leg. In the illustrated embodiment, the two outer legs have substantially the same lengths. Similarly, the middle leg 160 of the second E-core 112 is shorter than the first outer leg 162 and the second outer leg 164 by a second length difference LD2 (FIG. 6) such that an end surface 220 of the middle leg is closer to the inner surface 168 of the main body 166 of the second E-core than a respective end surface 222 of the first outer leg and a respective end surface 224 of the second outer leg.
When the middle leg 150 of the first E-core 110 and the middle leg 160 of the second E-core 112 are inserted fully into the passageway 122 of the bobbin 120, the end surface 212 of the first outer leg 152 of the first E-core abuts the end surface 222 of the first outer leg 162 of the second E-core. Similarly, the end surface 214 of the second outer leg 154 of the first E-core abuts the end surface 224 of the second outer leg 164 of the second E-core. The end surface 210 of the middle leg of the first E-core is adjacent to the outer surface 220 of the middle leg of the second E-core; however, the relative shortness of the respective middle legs with respect to the respective outer legs of the two E-cores causes a magnetic gap 230 to be formed between the opposing outer surfaces of the middle legs. The magnetic gap is a conventional air gap; however, the magnetic gap may be filled with a non-magnetic material, such as, for example, a polyester film.
The gap has a gap distance GD that is equal to the sum of the two length differences LD1, LD2 (e.g., GD=LD1+LD2). When the two length differences are the same, the gap distance is substantially equal to 2×LD1 or 2×LD2. The gap distance may also be formed by making either the first length difference LD1 of the first E-core or the second length difference LD2 of the second E-core equal to the desired gap distance and making the length of the middle leg of the other E-core equal to the lengths of the respective outer legs of the other E-core. Dividing the gap distance between the middle legs of the two E-cores allows the two E-cores to be identical or substantially identical.
The inductor 100 of FIGS. 1-7 operates in a conventional manner to provide a substantially constant inductance over a wide range of load conditions. For example, FIG. 8 illustrates a graph 400 of the DC bias characteristics of a conventional single-gap inductor such as the inductor of FIGS. 1-7. As illustrated by a curve 410 in FIG. 8, the conventional inductor has an inductance of approximately 3.5 millihenries over a wide range of DC bias currents from approximately 0 amperes to approximately 1.9 amperes. At a DC bias current of approximately 1 ampere, the inductance begins to decrease as the magnetic paths through the two E-cores start to saturate; however, the decrease is gradual as the DC bias current increases from approximately 1 ampere to approximately 1.9 amperes.
For some applications, an inductor having a variable inductance is desirable. For example, in a boost inductor circuit having a variable DC load, a relatively low inductance is desirable at heavy loads to reduce losses in the inductor and to allow switching at a higher frequency. When the boost inductor circuit is operating at a lighter load, a larger inductance is desired so that the circuit can switch at a lower frequency and thereby reduce losses in the circuit at the lighter load. The desired variable inductance has been achieved thus far by using a step-gap inductor such as, for example, described in U.S. Patent Application Publication No. 2010/0085138 to Vail, entitled “Cross Gap Ferrite Cores,” and in U.S. Pat. No. 9,093,212 to Pinkerton et al., entitled “Stacked Step Gap Core Devices and Methods.”
FIGS. 9-13 illustrate a basic step-gap inductor 500, which is derived from the conventional inductor 100 of FIGS. 1-7 by replacing the second E-core 112 of FIGS. 1-7 with a step-gap E-core 510. The other elements of FIGS. 9-13 generally correspond to the elements of the conventional single-gap inductor of FIGS. 1-7 and are numbered accordingly.
The step-gap E-core 510 is similar to the first E-core 110 and the second E-core 112 of FIGS. 1-7. The step-gap E-core comprises a middle leg 520, a first outer leg 522 and a second outer leg 524. The three legs extend from an inner surface 532 of a main body 530. In the illustrated embodiment, the first outer leg has an end surface 540 spaced apart from the inner surface of the main body by an outer leg length, and the second outer leg has an end surface 542 spaced apart from the inner surface of the main body by substantially the same outer leg length.
In the illustrated embodiment, the first side surface 180, the second side surface 182, the lower surface 184 and the upper surface 186 of the middle leg 520 of the step-gap E-core are numbered as described above for the middle legs 150, 160 of the first and second E-cores 110, 112.
Unlike the previously described middle leg 160 of the second E-core 112 in the embodiment of FIGS. 1-7, the middle leg 520 of the step-gap E-core 510 of FIGS. 9-13 has a two-part end surface 550. A first part 552 of the end surface of the middle leg is spaced apart from the inner surface 532 of the main body 530 of the step-gap E-core by a first length corresponding to the length of the middle leg of the embodiment of FIGS. 1-7. A first portion of the middle leg of the embodiment of FIGS. 9-13, which extends from the inner surface of the main body to the first portion of the outer surface, may have the second length difference LD2 (FIG. 12) relative to the lengths of the first outer leg 522 and the second outer leg 524 as described above. A second part 554 of the outer surface of the middle leg is spaced apart from the inner surface of the main body by a greater distance. The second portion of the middle leg is shorter than the lengths of the first outer leg and the second outer leg by a third length difference LD3 (FIG. 12). In the illustrated embodiment, the third length difference LD3 is less than the second length difference LD2.
As illustrated in the cross-sectional view in FIG. 13, when the first E-core 110 and the step-gap E-core 510 are inserted into the passageway 122 of the bobbin 120, the first part 552 of the outer surface 550 of the middle leg 520 of the step-gap E-core is spaced apart from the outer surface 180 of the middle leg 150 of the first E-core by a first gap distance GD1, which may be the same as the gap distance GD of the embodiment of FIGS. 1-7. The first gap distance GD1 is the sum of the first length difference LD1 (FIG. 6) and the second length distance LD2 (FIG. 12) as described above. The second part 554 of the outer surface of the middle leg of the step-gap E-core is spaced apart from the outer surface of the middle leg of the first E-core by a second gap distance GD2, which is the sum of the first length difference LD1 and the third length difference LD2. Thus, as illustrated in FIG. 13, the second gap distance GD2 is less than the first gap distance GD1. Accordingly, a step gap 560 is formed between the first E-core and the step-gap E-core. The step gap has a first gap portion 562 having the first gap distance GD1 and has a second gap portion 564 having the second gap distance GD2. In the illustrated embodiment, the first part and the second part of the outer surface of the middle leg have approximately the same surface areas; however, the surface areas may be different in other embodiments.
The step gap 560 of the inductor 500 of FIGS. 9-13 causes the inductor to have a greater variation in DC bias characteristics over a load range. The variation in the DC bias characteristics is illustrated by a curve 810 on a graph 800 in FIG. 14. The previously described curve 410 for the inductor 100 is also shown on the graph in FIG. 14 for comparison. As illustrated by the curve 810, the inductance at lighter current loads from approximately 0 amperes to approximately 0.6 ampere is fairly steady at approximately 6.5 millihenries with a gradual reduction to about 6.25 millihenries at 0.6 ampere. The decrease in inductance is faster as the current continues to increase above 0.6 ampere because the portions of the magnetic path affected by the shorter gap 562 become saturated and reduce the contribution of the magnetic path to the inductance. Because of the saturation of the portion of the magnetic path affected by the shorter gap, the inductance of the inductor of FIGS. 9-13 continues to decrease until the inductance of the step-gap inductor is approximately the same as the inductance of the conventional inductor 100 at approximately 0.95 ampere. As the load current continues to increase, the inductance of the step-gap inductor is less than the inductance of the convention inductor because the inductance is determined by the gap distance GD1 of the longer gap 564, which has approximately the same gap distance as the gap distance GD of the single gap 200 of FIG. 3, but has about one-half the surface area (or cross-sectional area) of the single gap. Accordingly, the magnetic path including the longer gap begins to saturate at lower currents and the inductance continues to decrease as shown by the curve 810.
Although the step-gap inductor 500 provides substantial benefits in providing a greater inductance at lighter load currents, a need exists for an inductor configuration that provides even greater inductance at lighter load currents and that provides a steady inductance at heavier load currents (e.g., does not exhibit the continued rapid reduction in inductance above 1.0 ampere as shown by the curve 810 in FIG. 14). Furthermore, a need exists for an inductor having such characteristics that can be formed without having to grind the end of one of the middle legs to form the step gap or having to form one the E-cores with a two-part middle leg with one part longer than the other part.